1. Technical Field
The present invention generally relates to biomedical devices such as ophthalmic lenses
2. Description of Related Art
Biomedical devices such as ophthalmic lenses made from siloxy-containing materials have been investigated for a number of years. Such materials can generally be sub-divided into two major classes, namely hydrogels and non-hydrogels. Hydrogels can absorb and retain water in an equilibrium state whereas non-hydrogels do not absorb appreciable amounts of water. Regardless of their water content, both hydrogels and non-hydrogels tend to have relatively hydrophobic, non-wettable surfaces.
Hydrogels represent a desirable class of materials for many biomedical applications, including contact lenses and intraocular lenses. Hydrogels are hydrated, crosslinked polymeric systems that contain water in an equilibrium state. Silicone hydrogels are a known class of hydrogels and are characterized by the inclusion of a silicone-containing monomer. Silicone hydrogels have typically been prepared by polymerizing mixtures containing at least one silicone-containing monomer and at least one hydrophilic monomer. Either the silicone-containing monomer or the hydrophilic monomer function as a crosslinking agent (a crosslinker being defined as a monomer having multiple polymerizable functionalities) or a separate crosslinker may be employed.
In the field of biomedical devices such as contact lenses, various physical and chemical properties such as, for example, oxygen permeability, wettability, material strength and stability are but a few of the factors that must be carefully balanced in order to provide a useable contact lens. For example, since the cornea receives its oxygen supply from contact with the atmosphere, good oxygen permeability is an important characteristic for certain contact lens material. Wettability also is important in that, if the lens is not sufficiently wettable, it does not remain lubricated and therefore cannot be worn comfortably in the eye. Accordingly, the optimum contact lens would have at least both excellent oxygen permeability and excellent tear fluid wettability.
An advantage of silicone hydrogels over non-silicone hydrogels is that the silicone hydrogels typically have higher oxygen permeability due to the inclusion of the siloxy-containing monomer. Silicone-containing monomers for use in the formation of silicone hydrogels are well known and numerous examples are disclosed in, for example, U.S. Pat. Nos. 4,136,250; 4,153,641; 4,740,533; 5,034,461; 5,070,215; 5,260,000; 5,310,779 and 5,358,995. However, one problem associated with silicone lenses is the surfacing of silicone chains which create hydrophobic areas on the lens. This will adversely impact wettability, on eye-movement and comfort to the user.
Karunakaran et al., “Synthesis, Characterization, and Crosslinking of Methacrylate-Telechelic PDMAAm-b-PDMS-b-PDMAAm Copolymers”, Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 45, pp. 4284-4290 (2007) (“Karunakaran et al.”) discloses the preparation of a new amphiphilic methacrylate-telechelic pentablock copolymer by reversible addition fragmentation chain transfer (“RAFT”) polymerization. As shown in Scheme I in Karunakaran et al., a polysiloxane monomer comprising one or more thio carbonyl fragments of a RAFT agent (2) is used as an intermediate in the preparation of the amphiphilic methacrylate-telechelic pentablock copolymer. Karunakaran et al. further discloses that the new amphiphilic methacrylate-telechelic pentablock copolymers can be used in an ophthalmic application such as in the formation of a contact lens. However, the process for making the amphiphilic methacrylate-telechelic pentablock copolymers is time consuming and employs different reagents and process conditions. This, in turn, can cause reproducibility problems. In addition, the methacrylate-telechelic copolymers prepared by Karanakaran et al. are cross-linking agents, which can increase the “effective” cross-link density of the resulting product resulting in a higher modulus of the product.
Accordingly, there remains a need to provide improved silicone hydrogels that are soft enough to make soft contact lenses, which possess high oxygen permeability, suitable water content, and sufficient elasticity, and are comfortable to the contact lens wearer. It would also be desirable to provide improved silicone hydrogels that are easy to manufacture in a simple, cost effective manner.